feat: add post story command

[howwohmm]

Feature Request

What problem does this solve?

Currently there's no way to post Instagram Stories from the CLI. Users who build or manage assets locally have to switch to a phone/browser to post them — breaking the terminal workflow.

Proposed solution

Add an instagram-cli post story [<username>] command with a 3-screen TUI:

1. File browser — navigate local filesystem, pick a .jpg, .jpeg, .png, or .mp4 file
2. Audience picker — toggle Everyone ↔ Close Friends
3. Confirmation — review file, type, audience before posting

Uses ig.publish.story() from instagram-private-api. No new dependencies required.

Notes

• Video cover image uses a 1×1 black JPEG placeholder to avoid an ffmpeg dependency (v1 limitation documented in the confirmation screen)
• Close friends posts use toBesties: true
• I have a working implementation ready to submit as a PR